Neurodegeneration affects an estimated 50 million Americans each year, significantly reducing the quality of life for the patients and their families and costing hundreds of billions of dollars in health care. One of the central players in neurodegeneration is alpha synuclein (a-syn), the major component of the Lewy bodies seen in Parkinson's disease (PD) patients. Aberrant a-syn folding has been implicated in multiple forms of neurodegeneration and recently shown to result in self-propagating prion-like action causing multiple system atrophy (MSA) in humans. While numerous factors are believed to contribute to a-syn misfolding in different diseases, the mechanisms that trigger a-syn transition from a normal to pathological state are not understood. Our preliminary data demonstrate that a-syn in the normal brain undergoes arginylation, an emerging posttranslational modification recently shown to be essential for protein homeostasis and protein-protein interactions. We find that arginylation prevents the pathological aggregation of a-syn in neurons and that transgenic mice lacking arginyltransferase Ate1 in the brain develop symptoms of neurodegeneration. Consistent with these data, human PD patients show a pronounced and significant loss of Ate1 expression in dopaminergic neurons. These results drive our central hypothesis that a-syn arginylation facilitates normal a-syn homeostasis and that lack of a-syn arginylation leads to abnormal a-syn accumulation thereby promoting neurodegeneration. We will test this hypothesis through the following set of integrated specific aims that will build on the diverse expertise of our collaborative research team: 1) test the role of a-syn arginylation in regulation of a-syn aggregation and neuropathology in neurons; 2) uncover the role of arginylation in neurodegeneration in mouse models; and 3) establish a mechanistic link between arginylation, a-syn, and PD pathology in human patients. Together, the proposed work will characterize a novel molecular mechanism that potentially underlies PD and a-syn dependent neurodegeneration and will develop new avenues for PD diagnosis and treatment.